According to a known technique, material can be broken by subjecting it to an impulse loading. An impulse loading of this kind is created by allowing the material to collide with a wall at high speed. It is also possible, in accordance with another option, to allow particles of the material to collide with each other. The impulse loading results in microcracks, which are formed at the location of irregularities in the material. These microcracks continuously spread further under the influence of the impulse loading until, when the impulse loading is sufficiently great or is repeated sufficiently often and quickly, ultimately the material breaks completely and disintegrates into smaller parts. Depending on the specific material properties of the collision partners, in particular the mechanical properties, such as the elasticity, the brittleness and the toughness, and the strength, in particular the tensile strength, on the one hand of the material which collides with an impact face of an impact member at great speed and on the other hand of the material which forms the said impact face, these materials become deformed or yield during the impact. In any case, the impact loading always results in deformation and wear to both collision partners. The impact face can be formed by a hard metal face or wall, but also by grains or a bed of its own material. The latter case is an autogenous process, and the wear during the impact remains limited.
The movement of the material is frequently generated under the influence of centrifugal forces. In this process, the material is flung away from a quickly rotating rotor, in order then to collide at high speed with an armoured ring which is positioned around the rotor and optionally rotates about a vertical shaft in the same or the opposite direction. If the aim is to break the material, it is a precondition that the armoured ring be composed of harder material than the impacting material; or is at least as hard as the impacting material. The impulse forces generated in the process are directly related to the velocity at which the material leaves the rotor and strikes against the armoured ring. In other words, the more quickly the rotor rotates in a specific arrangement, the better the breaking result will be. Furthermore, the angle at which the material strikes the armoured ring has an effect on the breaking probability. The same applies to the number of impacts which the material undergoes or has to deal with and how quickly in succession these impacts take place. This method is known from various patents and is employed in a large number of devices for breaking granular material or making it collide.
Since about 1850, many hundreds of patents have been granted worldwide for this method. A distinction can be drawn here between single impact crushers, in which the material is loaded by a single impact, indirect multiple impact crushers, in which the material is accelerated again after the first impact and loaded by a second impact, which process can be repeated further, and direct multiple impact crushers, in which the material is loaded in immediate succession by two or more impacts. Direct multiple impact is preferred, since this considerably increases the breaking probability.
A single impact crusher, intended for breaking granular material, was announced in the literature as early as 1870 (Ritter Von Rittinger, Lehrbuche der Aufbereitungskunde, FIG. 34), the crusher being equipped with a rotor on which are located relatively long guides, by means of which the material is accelerated and then flung outwards, at great speed, from the delivery end of the guides against a knurled, stationary armoured ring, which is disposed around the rotor, during which impact the material, if the velocity is sufficiently great, breaks. In the known device for breaking material by means of a single impact, the material to be broken is flung outwards, under the effect of the centrifugal forces, on rotation of the rotor. The velocity obtained by the material in the process is generated by guiding the material outwards along a guide, and is composed of a radial velocity component and a velocity component which is directed perpendicular to the radial component, in other words a transverse velocity component.
The theory of the single impact crusher was described extensively as early as 1889 (M. E. Bordier; Broyeur Vapart; Revue de L'Exposition de 1889, septieme partie, Tome II, Les machines-outils. Travail des divers Materiaux. Broyeurs, concasseurs, pulverisateurs, etc., p. 627-631, 1889). When viewed from a stationary position, the take-off angle of the material to be broken from the edge of the rotor blade is determined by the magnitudes of the radial and transverse velocity components which the material possesses at the moment when it comes off the delivery end of the guide. If the radial and transverse velocity components are equal, the take-off angle is 45.degree.. Since in the known single impact crushers the transverse velocity component is generally greater than the radial velocity component, the take-off angle is normally less than this, and lies between 35.degree. and 45.degree.. Over the relatively short distance covered by the material to be broken in the known devices until it strikes the impact face, the force of gravity, the air resistance, any air movements and a self-rotating movement of the grains normally have no significant effect on the direction of movement for (mineral) grains with diameters of greater than 5 mm. For grains with a smaller diameter, or grains composed of lighter material, the effect of the air resistance, in particular, increases considerably. As a general rule, it can be stated that the effect of the air resistance increases for grains of smaller diameter, while the effect of the grain configuration on the air resistance increases for grains of larger diameter. The known atmospheric impact crushers can be used to process material to a diameter of 1 to 3 mm. For smaller diameters, the breaking process has to take place in a chamber in which a partial vacuum can be created.
As long as the diameter is not too small, the material to be broken therefore moves, when seen from a stationary viewpoint, at a virtually constant velocity along a virtually straight line towards the location of the impact on the stationary armoured ring. The impact angle of the granular material against this armoured ring is defined by the take-off angle of the granular material from the delivery end of the guide and by the angle at which the impact face is disposed at the location of the impact.
In the known single impact crusher, the impact faces are generally disposed in such a manner that the impact in the horizontal plane as far as possible takes place perpendicularly. The specific arrangement of the impact faces which is required for this purpose means that the armoured ring as a whole has a type of knurled shape. A device of this kind is known from U.S. Pat. No. 5,248,101. The stationary impact faces of the known devices for breaking material are frequently of straight design in the horizontal plane, but may also be curved, for example following an involute of circle. A device of this kind is known from U.S. Pat. No. 2,844,331. This achieves the effect of the impacts all taking place at an impact angle which is as far as possible identical (perpendicular). U.S. Pat. No. 3,474,974 has disclosed a device for single impact in which the stationary impact faces are directed obliquely downwards in the vertical plane, with the result that the material is guided downwards after impact. This results in the impact angle being more optimum, while the impact of subsequent grains is affected to a lesser extent by fragments from previous impacts, which is known as interference.
The problem with the known single impact crusher described is that the comminution process takes place during one single impact which is directed as perpendicularly as possible. Examinations have shown that a perpendicular impact is not optimum for comminuting most materials by means of impact loading and that a greater breaking probability can be achieved, depending on the specific type of material, with an impact angle of approximately 75.degree., or at least between 70.degree. and 85.degree.. Furthermore, the breaking probability can be increased considerably further if the material for breaking is subjected to an impact loading not just once, but rather a number of times in quick succession, and at any rate at least twice.
Furthermore, in the impact crusher described, the impact of the granular material is to some extent considerably disturbed by the projecting corners of the impact plates. This interference can be given as the length which is calculated by multiplying the diameter of the fragments of material for breaking by the number of projecting corners of the armoured ring, with respect to the total length or the periphery of the armoured ring. In the known single impact crushers, frequently more than half the grains are interfered with during impact. This interference increases considerably as the corners of the impact plates become rounded by wear; with the result that even the beneficial effect of directing the impact faces obliquely forwards and making them curved is quickly cancelled out.
The single impact, the impact angle which is as far as possible perpendicular, and the disturbing influences resulting from interference and above all from the projecting corners are the cause of the fact that the breaking probability of the known device described for breaking material by a single impact is limited, while the quality of the broken product can exhibit considerable variations. To achieve a reasonable degree of comminution, it is frequently necessary to increase the impact velocity, which requires extra power and causes the wear to increase considerably, while an undesirably high content of extremely fine particles may result.
DE 1,253,562 has disclosed a device for breaking grains by means of a single impact in which use is made of two rotor blades situated one above the other, which are both provided with guides and both rotate in the same direction, at the same angular velocity and about the same axis of rotation. In this device, a first part of the material is accelerated onto the upper rotor blade and is flung outwards against a first armoured ring which is disposed around the upper rotor blade. The second part of the material is accelerated onto the second rotor blade, which is situated below the first rotor blade, and is flung against a second armoured ring, which is disposed around this rotor blade. The capacity is thus doubled, as it were. DE 1,814,751 has disclosed a device in which more than two systems are placed above one another.
Various patents have disclosed methods for accelerating granular material onto a rotor, the attempt being to achieve the required velocity while consuming as little power as possible and above all to limit the wear as far as possible.
U.S. Pat. No. 3,955,767 has disclosed a device by means of which the material is accelerated by guide members which are provided with relatively long rotating radial guide faces. This process has the advantage that these grains are able to make good contact with the guide face and are flung outwards from the delivery end of the guide member at approximately the same velocity and at approximately the same take-off angle. However, the wear to these relatively long guides is extremely high; this is because this wear increases very progressively, to the third power of the radial distance, as the velocity increases.
In addition to radially directed guides, devices are also known in which the guides are not disposed radially, but rather are curved forwards or backwards, when seen in the direction of rotation, and may even be of double-curved design. UK 309,854 has disclosed a device in which the guides are bent backwards and the curvature is integrated with the curvature of stationary impact faces. UK 1,434,420 has disclosed a device in which the guides are designed in the form of a so-called scoop. EP 0,191,696 has disclosed a device in which the guides are bent forwards, in such a manner that the material itself attaches to the guide face under the influence of centrifugal force, so that an autogenous guide face is formed. U.S. Pat. No. 1,875,817 has disclosed a device in which rotating hammers are disposed along the outside of the rotor blade, by means of which hammers the material is flung against stationary impact plates. Symmetrical arrangements are also known, such as from U.S. Pat. No. 1,499,455 and EP 0,562,194, which make it possible to allow the device to function rotating both forwards and backwards. UK 2,092,916 has disclosed a device in which the guide is designed in the form of a tube. It has been found that changing the form of the longitudinal direction of the guide face in general has a relatively limited effect on the wear and the power consumption, because it is, after all, necessary to achieve a certain velocity, at which the material to be broken is flung away and strikes the stationary impact member.
U.S. Pat. No. 4,787,564 has disclosed a guide member in which the guide face is perforated, so that the material is directed better and, at the same time, is guided outwards at various levels situated parallel and next to one another.
WO 96/32195, in the name of the applicant, has disclosed a rotor-blade design in which the guides with the central feed are disposed at various levels, while the discharge ends lie more towards the outside and at the same level. This means that the number of guides on the rotor blade, and thus the capacity, can be doubled without the feed of the material to the central feed of the various guide members being impeded.
U.S. Pat. No. 5,184,784 has disclosed a method for accelerating granular material, in which guide shoes, in the form of projections, are disposed on the edge of a rotor blade, relatively far away from the axis of rotation. Thus the granular material, which is metered onto the centre of the rotor and, from there, spreads outwards over the rotor blade without hindrance, is taken up at a relatively great velocity, accelerated and flung outwards. This type of rotor, which exhibits less wear than a rotor which is equipped with longer, radially directed guides, which extend from the central part to the edge of the rotor blade, is in practice in widespread use in single impact crushers. The rotor blade of the known method, having the projections, does, however, exhibit the drawback that the acceleration takes place in a very uncontrolled manner. Grains can be taken up at the corners on the inside or the outside of the projection or anywhere along the face, and from there can be loaded by means of an oblique or perpendicular impact and flung away; however, and this frequently occurs, they can also be accelerated by being guided along (a section of) the face of the projection, while combinations, in particular of an oblique impact followed by the partial guidance, are also possible. In these known methods, the grains are consequently flung outwards at extremely changeable and divergent velocities in various directions, while the wear to the guides is still in relative terms extremely high, in particular owing to impact friction and above all guide friction. Owing to the uncontrolled acceleration, the impacts of the various grains against the stationary, knurled armoured ring take place at very different velocities and at various angles. To achieve a reasonable level of comminution, the rotational speed of the rotor has to be adapted to the grains which have the lowest breaking probability, which strike against the armoured ring at the most unfavourable angle and at the lowest velocity. The rotational speed therefore has to be relatively high. The broken product thus exhibits a considerable spread in grain size distribution, frequently with a high content of undesirable, very fine constituents, while the power consumption and also the wear are still relatively high. U.S. Pat. No. 3,174,698 has disclosed a single impact crusher in which round bars are mounted instead of projections. The metering face is formed by a relatively steep cone, the intention being to allow the material to strike the round bars at a high velocity, so that the grains can break even during this impact, after which the fragments are flung outwards against the stationary armoured ring. The symmetrical arrangement of the bars makes it possible to allow the rotor blade to rotate in both directions.
It is important that the material should be metered as evenly as possible onto the metering face on the centre of the rotor. It is necessary to avoid metering the material at excessive velocity or from an excessive height. EP 0,740,961 has disclosed a device in which a metering chamber is disposed above the inlet of the rotor, from which metering chamber the material is metered onto the central part of the rotor blade in a uniform manner.
Methods are also known in which the granular material is accelerated not in one step, as in the above-described discovered methods for single impact, but rather in two steps, by means of guidance.
U.S. Pat. No. 3,032,169 has disclosed a device for accelerating granular material, by means of which the grain particles are guided from the central part of the rotor blade with a relatively short preliminary guidance to longer guides disposed directly radially on the outside; the material is accelerated along these longer guides and then flung against a stationary, knurled armoured ring disposed around the rotor blade. The object of the invention is to guide the grains, with the aid of the short preliminary guides, in a more regular distribution to the longer guides, specifically in such a manner that the grains do not strike these longer guides, but rather are accelerated along them, as far as possible by means of guidance, in order then to be flung outwards from the delivery end.
U.S. Pat. No. 3,204,882 has disclosed a device for accelerating granular material, by means of which the granular material is guided, by means of a preliminary guide disposed tangentially directly along the central part of the rotor blade, to the guide face of a guide shoe, which guide face is directed more or less at 90.degree. outwards and is disposed at the end of the first tangential preliminary guide. This design aims to prevent the granular material from striking the guide surface of the shoe structure with an impact, instead of which it is to be accelerated along the guide surface in a regular manner and as far as possible in a sliding movement, in order then to be flung outwards, past the delivery end of the guides, against a knurled armoured ring. It is stated that this method considerably reduces the wear and that the granules are accelerated more regularly. However, the wear to the guide face of the guide shoe is still high. Impact plates are additionally arranged behind the shoe structure, by means of which impact plates material or grain fragments which rebound after impact against this stationary armoured ring are collected and loaded again. These impact plates can also be designed as impact hammers and at the same time serve as a protective structure for the rotor.
Instead of a metal guide face, the material on the rotor blade can also be accelerated along a bed of the same material, i.e. an autogenous guide face. For this purpose, the rotor blade has to be equipped with a structure in which this same material accumulates under the effect of centrifugal force and forms an autogenous guide bed, in which case the structure in question is a chamber vane structure.
U.S. Pat. No. 1,547,385 has disclosed a single impact crusher in which the material becomes attached to the rotor blade along sections of a circular wall, the material being accelerated and then flung outwards, primarily in a tangential direction, through openings in the cylinder wall, primarily with the tip velocity at that location. The amount of material which is guided outwards through the slot-like openings in the cylinder wall, that is to say the flow rate, is determined primarily by the radial velocity component which the material has at the moment at which it passes through the slot-like opening. On the baseplate of the cylindrical chamber, where the contact with the grains is limited, the material only develops a low radial velocity, with the result that the flow rate also remains limited; moreover, it is only affected to a limited extent by the angular velocity. A further problem with the known structure is that the material becomes attached to the cylindrical wall section between the slot-like openings, so that bridges can easily be formed, so that the flow of the granular material outwards is considerably impeded. The manner in which the grains are guided outwards through the openings in the cylinder wall is extremely chaotic, because essentially there is an absence of any form of guidance. Another problem is presented by the considerable wear which occurs along the walls of the slot-like opening. U.S. Pat. No. 1,405,151 has disclosed a similar design, in which the openings (delivery end) in the cylinder walls are provided with guide projections, so that an autogenous guide face can be formed. This design is improved further in U.S. Pat. No. 4,834,298, so that a tangentially directed, autogenous guide face can be formed in the cylinder.
WO 96/20789 has disclosed a device in which the material on the centre of the rotor blade is taken up in a sleeve, from where it is flung outwards along the top edge, under the influence of centrifugal force. It is claimed that this considerably limits the wear. U.S. Pat. No. 3,834,631 has disclosed a design in which the cylinder is arranged in tumbling fashion. JP 61-216744 has disclosed a symmetrical rotor-blade structure which has the form of a cone which widens downwards. The material is introduced from above onto a co-rotating distributor disc which is suspended in the top of the cone and, from there, is flung outwards, where the material becomes "attached" to the inside of the cone in vane structures which are arranged there. In these structures there is formed an autogenous guide bed which is, as it were, inverted and along which the material is accelerated and flung outwards along the bottom of the edge of the cone.
U.S. Pat. No. 3,174,697 has disclosed a device for accelerating granular material, in which the rotor is equipped with a guide, each in the form of two chamber vanes which are positioned in line with one another. Under the influence of centrifugal force, the granular material accumulates in these chamber vanes, resulting in the formation of a type of bent, tangentially directed, autogenous guide face, along which the granular material is accelerated and flung outwards.
U.S. Pat. No. 3,162,386 has disclosed a similar device for accelerating granular material with guide arms which are directed radially outwards and along which guides more than one vane structure is fastened, each of which is disposed tangentially in such a manner that the granular material accumulates in these vanes under the influence of centrifugal force, with the result that the vanes as a whole form an autogenous bed of grains, along which the granular material is accelerated and flung outwards by stepwise guidance. This combination aims to prevent the material from rubbing too much against the rotor blades, due to the fact that the fillet-like top ends of the fillings in the chamber vanes as a whole form an autogenous guide face, along which the material is accelerated and guided outwards. The number of chamber vanes is determined by the diameter of the rotor. At the same time, the wear to the guides, and in particular to the rotor, is limited. This is because the vanes are designed in such a manner that the granular material is prevented from rubbing along the bottom plates and top plates of the rotor housing, as a result of which wear to these plates is prevented. In a supplementary U.S. Pat. No. 3,346,203, a protective structure is also provided for the device of this invention, which structure is arranged in the form of pins along the edge of the rotor, between the upper and lower blades, thus preventing granular material which rebounds after it has struck the stationary armoured ring from damaging the rotor-blade structure. The known crusher brings about a certain degree of direct, multiple autogenous impact, albeit uncontrolled. Since the "impact face" essentially functions as the subsequent guide face, this action is ineffective.
EP 0,101,277 has disclosed a method for accelerating granular material and making it collide, using guides which are disposed virtually tangentially and, furthermore, are designed such that an autogenous guide face made of the same material is formed against these guides, under the influence of centrifugal force. The known structures, by means of which an autogenous guide face is formed, aim to limit wear. However, a relatively great amount of wear occurs at the delivery end of a guide of this kind. Moreover, the tangential arrangement of the guide is the cause of the fact that the radial velocity component is used only to a very limited extent for accelerating the material. The grains come off the delivery end with essentially only the tip velocity and scarcely any radial velocity. As a result, much of the added energy, approximately half, is lost. Furthermore, a large quantity of energy is lost because the grains in the rotor are guided towards the edge of the rotor in an essentially unnatural, forwards movement. Consequently, the known rotor structure has only a limited efficiency. A major problem with the known crushers is that because the grains do not develop any radial velocity along the guides, they do not have any outwards velocity, when seen from the viewpoint which moves together with the delivery end, when they come off the delivery end of the guide, and therefore they move directly backwards, seen in the direction of rotation, and cause intense wear along the outer edge of the delivery end (tip). Thus, moreover, considerable velocity is lost. Dozens of tip designs are known for the delivery end of rotors of this kind, which designs aim to limit the wear, and are known inter alia from U.S. Pat. No. 5,131,601 and EP 0,187,252, EP 0,265,580 and EP 0,452,590, UK 2,214,107 and WO 95/10358, WO 95/10359 and WO 95/11086. However, none of the known tip designs functions satisfactorily, and they are unable to prevent the occurrence of intense wear at the delivery end. U.S. Pat. No. 4,390,136 has disclosed a device in which the guide, which is of symmetrical design, is formed by vertical bars, which are disposed along the edge of the rotor blade in such a manner that a type of semi-autogenous guide face is produced.
The material is flung from the rotor against an armoured ring disposed around the rotor, during which impact the material breaks. It is possible to combine the guide and impact structures in various ways: a steel guide face and a steel impact face, known as steel-on-steel, an autogenous guide face and a steel impact face, known as stone-on-steel, an autogenous guide face with an autogenous impact face, known as stone-on-stone, and a steel guide face with an autogenous impact face, known as steel-on-stone.
The armoured ring is generally formed by separate elements, i.e. impact plates, which are disposed around the rotor blade with their impact face directed perpendicular to the straight path which the grains describe when they are flung outwards from the rotor blade. The wear to the impact plates is relatively high, since the grains continuously rub along them at high speed. U.S. Pat. No. 4,090,673 has disclosed a typical structure (steel-on-steel) in which the separate impact plates are provided with a special fastening structure, so that they can be exchanged quickly. JP 2-237653 has disclosed a device in which the impact faces are designed such that less hindrance is undergone as a result of the wear of the projecting corners. EP 0,135,287 has disclosed a design in which the impact plates comprise elongate, radial blocks which are disposed next to one another around the rotor blade. These blocks, as they become worn, can always be moved forwards, so that they have a longer service life. In this case, the impact face of the armoured ring is knurled centrally and is no longer directed perpendicular to the path which the grains describe. Overall, it has to be stated that in the known crushers the wear is relatively high in relation to the intensity of comminution.
JP 06000402 and JP 06063432 have disclosed devices in which the impact plates are vertically adjustable, so that the wear can be spread more evenly along the impact face.
JP 06091185 has disclosed a device which is symmetrical and in which it is possible to charge the length of the guide members in the radial direction and to adjust the height of the impact faces. This document contains an extensive (theoretical) discussion of the movement of granular material along a radially disposed guide face.
Instead of an armoured ring, against which the material is flung from the delivery end of the autogenous guide, a trough structure may be disposed around the edge of the rotor, in which trough an autogenous bed of the same material builds up, against which bed the granular material which is flung off the rotor blade then strikes (stone-on-stone). U.S. Pat. No. 4,575,014 has disclosed a device with an autogenous rotor blade, from which the material is flung against an armoured ring (stone-on-steel) or a bed of the same material (stone-on-stone). JP 59-66360 has disclosed a device in which the material is flung from steel guides onto an the same bed (steel-on-stone). Comminution takes place in the bed of the same material by the grains colliding with one another and undergoing friction. As a result, the wear is limited further; however, the impact intensity, i.e. the impulse loading of the grains in the autogenous ring, is limited in the known method. Due to the fact that primary the transverse velocity component (tip velocity) is active and the radial velocity component, although limited, is variably active, the grains are guided into the autogenous bed at extremely shallow but very diverse angles (from approximately 5.degree. to 20.degree.). Consequently, the impact against the autogenous bed of the same material takes place at a very oblique, and moreover variable impact angle, which as a result has limited effect. As a result, the grains are guided in a movement "running round" along the autogenous bed. When the grains collide with one another, the impacting grains are loaded against grains which continue to move along the said bed of the same material; i.e., as it were, from behind, which also has little effect. The level of comminution of the known method is therefore low, and the crusher is primarily employed for the after-treatment of granular material by means of rubbing the grains together, and in particular for "cubing" irregularly shaped grains. A further drawback is that if the material for breaking contains fine material, or a large number of small particles are formed during the autogenous treatment, the autogenous bed can easily become blocked, forming a so-called dead bed of fine particles. Material which strikes against and rubs along, a dead bed of this kind is relatively ineffective. It is therefore in actual fact not possible to call this a comminution process, but rather a more or less intensive after-treatment process for material which has already been broken.
JP 04300655 has disclosed a single impact crusher in which the autogenous ring is designed so that it can be emptied at the bottom, thus allowing the bed of the same material to be, as it were, exchanged regularly. As a result, a dead bed is less likely to form. U.S. Pat. No. 4,844,364 has disclosed a single impact crusher in which the autogenous bed is formed in a structure in which it can move right round, thus aiming to make the autogenous action more intensive.
JP 07275727 has disclosed a single impact crusher in which an armoured ring is disposed around part of the rotor and a bed of the same material is disposed around part of the rotor, so that the intensity of comminution differs considerably and a grain size distribution with a large dispersion can be achieved.
EP 0,074,771 has disclosed a method for breaking material using autogenous guides and a stationary bed of the same material, in which part of the granular material is not accelerated but rather is guided around the outside of the rotor. Two streams of grains are thus formed, a horizontal first stream of grains, which is flung outwards onto the rotor from the guides, and a vertical second stream of grains which, as it were, forms a curtain of granular material around the guides. The material from the first accelerated horizontal stream of grains now collides with the material of the second, unaccelerated vertical stream of grains, whereupon the two collided streams of grains are taken up in an autogenous bed of the same material, so that this can be known as an inter-autogenous comminution process. This method, which aims to save energy and to reduce the wear, has a number of drawbacks. The loading takes place by the perpendicular collision between a grain moving quickly in the horizontal direction and a grain moving relatively slowly in the vertical direction. The effectiveness of a collision of this kind is essentially low; in the most favourable scenario, when grains of the same mass hit each other full on, at most half of the kinetic energy is transmitted, while only a limited fraction of the grains actually contact each other fully. Furthermore, the material which is accelerated with the guide is concentrated in separate first horizontal streams of grains, which are guided, from the guides, around the inside of a vertical curtain, or second stream of granular material. Consequently, the grains from the second stream of grains are not all loaded uniformly. In fact some of the grains from the second stream of grains are not even touched at all before being collected at the bottom in the bed of the same material. The specific, very oblique angle at which the grains from the first stream of grains leave the rotor blade is furthermore the reason for the intensity of the impact of the collided material from the first and second streams of grains against the autogenous bed of the same material being limited. The effectiveness of the known method is therefore limited. Here too, a dead autogenous bed is easily formed, as a result of which the autogenous action along the bed of the same material is limited. Moreover, the method is extremely susceptible to changes in the quantitative distribution of the material across the first and second streams of grains.
U.S. Pat. No. 3,044,720 has disclosed a device for indirect multiple impact, in which the material is flung, with the aid of a first rotor blade, against a first stationary armoured ring where, after impact, it is taken up and guided to a second rotor blade situated beneath the first, which rotates at the same angular velocity, in the same direction and about the same axis of rotation as the first rotor blade, on which second rotor blade the second part of the material is accelerated for the second time, frequently at greater velocities than during the impact against the first impact face, and flung against a second stationary armoured ring, which is disposed around this second rotor blade. U.S. Pat. No. 3,160,354 has disclosed methods in which this process is repeated a number of times, or at least more than twice. U.S. Pat. No. 1,911,193 has disclosed a device in which the impact plates on the rotor blade situated at a lower level are disposed ever further from the axis of rotation, so that the impact velocity increases.
DE 38 21 360 (JP 0596194) has disclosed a method for indirect multiple impact, in which the material, after it has been accelerated for the first time on a first rotor blade and flung against an armoured ring, is taken up on a second rotor blade, situated below the first, from where it is flung against an autogenous bed of the same material. JP 08192065 has disclosed a similar device, in which the material is flung from both the first and the second rotor blades against a bed of the same material. This structure aims, inter alia, to utilize as much as possible of the kinetic energy which the grain still possesses after the first impact. However, this kinetic energy is generally limited, since the material often loses virtually all its kinetic energy during the stationary impact and, as it were, kills this energy. In order to prevent the formation of a dead bed in the autogenous ring, air can be injected into the trough structure from below, so that relatively fine particles can be blown out of the material bed.
Indirect multiple impact of this kind can achieve a high level of comminution. However, the wear and the power consumption are high, while it is frequently difficult, after the first impact, to guide the material uniformly to the next rotor blade, on which the material is accelerated again and undergoes a second impact.
WO 94/29027, which is in the name of the applicant, has disclosed a device for direct multiple impact, the impacts taking place in an annular and slot-shaped space between two casings which are positioned one above the other and are in the form of truncated cones which widen downwards and which are both rotatable in the same direction and at the same angular velocity as the rotor, around the same axis of rotation. Instead of cones, in the known method for direct multiple impact, the impact faces can also be composed of straight faces which are disposed in the centre before the delivery end of the guides and, in the horizontal plane, are directed perpendicular to the radius of the rotor. This angle which is directed perpendicularly in the horizontal plane may be altered by +10.degree. and -10.degree., thus allowing the material which is to be broken to be guided downwards between the impact faces as far as possible perpendicularly in a zig-zag path of direct multiple impact, and making it possible to prevent the material to be broken from striking the side walls of the breaking chamber. In the rotating breaking chamber, primarily the radial velocity component is utilized; the residual energy, which is mostly transverse, is only utilized after the material is guided out of the rotating breaking chamber and strikes stationarily disposed impact faces.
Instead of being stationary, the impact face may also be designed to rotate, about the same axis of rotation as the rotor blade. In this case, rotation can take place in the same direction and at the same angular velocity as these guides, but also oppositely thereto.
UK 376,760 has disclosed a method for breaking granular material, by means of which a first and a second part of the granular material are flung outwards, with the aid of two guides which are situated directly above one another, are directed towards one another and rotate around the same axis of rotation but in opposite directions. As a result, the two streams of grains are oppositely directed, with the result that the grains hit each other at a relatively great velocity and are then taken up in a trough structure which is disposed around the two rotor blades and in which the granular material builds up a bed of the same material. In order to allow the grains to hit each other correctly, it is necessary to concentrate the oppositely directed streams of grains as far as possible in one plane between the rotor blades. With guides, this can be achieved only to a limited extent, because the grains, when they come off the delivery end, under the influence of centrifugal force, immediately move outwards in a horizontal path. Therefore, only a limited fraction of the grains actually collide fully with one another. The specific arrangement of the guides, which is necessary in order as far as possible to move the streams of grains into one plane when they come off the delivery end of the guides is the reason for the wear to the guides being relatively great. JP 2-227147 has disclosed a similar structure in which the material is launched from a symmetrical autogenous structure.
JP 2014753 has disclosed a device in which the material on a rotor, which is equipped with autogenous guides, is flung outwards against an autogenous bed of the same material, which is formed in a trough structure which rotates in the same direction as the rotor, but is driven separately.
DE 31 16 159 has disclosed a device in which an autogenous ring is disposed around a sleeve structure in the centre of the rotor blade, which autogenous ring rotates in a direction opposite to that of the sleeve structure.
JP 2-122841 has disclosed a device in which a rotor is disposed in the centre, which rotor is provided with first chamber vanes, in which material accumulates, forming a guide face, around which is disposed a rotor with similar, second chamber vanes which rotate in the opposite direction and from which the material is flung into the autogenous bed disposed around it. The material is flung from the first chamber vane at great velocity against the material in the second chamber vane and, from there, into the stationary autogenous ring. A problem with the known crusher is the transfer from the first to the second chamber vane, which is impeded to a considerable extent by the edges of the chamber vanes.
JP 2-122842 has disclosed a device in which a ring structure is disposed around the outside of the rotor with chamber vanes, which rotor is disposed in the centre, which ring structure rotates in the opposite direction and an autogenous bed accumulates therein.
JP 2-122843 has disclosed a crusher, of which two rotors are disposed in the crusher chamber, which are provided with two rotors, which are positioned one above the other, rotate in opposite directions about the same shaft and are each provided with chamber vanes, the material being guided outwards into the autogenous ring in two oblique paths which are situated one above the other and in opposite directions, which process leads to an intense after-treatment. A disadvantage is that the jets do not immediately contact one another, but rather do so only after they have struck the autogenous bed.
A significant problem with the known rotors operating in opposite directions is the complicated separate drive.
SU 797761 has disclosed a device in which the material, after it has been accelerated on the rotor blade, is flung outwards against a stationary, knurled edge, from where it is taken up again by projections which are fastened along the edge of the rotor. However, this process, which is known as direct multiple impact, is disrupted by the material not rebounding "cleanly" when it strikes the points of the knurled edge and not being taken up by the projections.
DE 39 26 203 has disclosed a rotor structure in which rebound plates are disposed behind the chamber vanes for taking up material which rebounds from the armoured ring, i.e. direct multiple impact. JP 06079189 has disclosed a similar, but symmetrical design for indirect multiple impact, the rebound plates being fastened in a pivoting manner along the outer edge. U.S. Pat. No. 2,898,053 has disclosed a direct multiple impact crusher in which the material, after it has struck a stationary armoured ring from the rotor blade, is taken up by impact plates which are suspended along the bottom of the rotor blade.
DE 39 05 365 has disclosed a direct multiple impact crusher, by means of which the material is guided from the rotor blade between impact faces which are directed radially outwards, are positioned next to one another and are disposed around the rotor blade. The material executes a zig-zag movement between these impact plates. A problem with the known impact crusher is the disruption from the points of the impact plates.
EP 0 702 598, which is in the name of the applicant, has disclosed a direct multiple impact crusher, by means of which the material, after it is flung from the rotor blade, is taken up in a circular, gap-like space which is disposed around the rotor blade and in which the material is guided downwards in a zig-zag path. This crusher functions only if the distance between the edge of the rotor blade and the surrounding stationary impact face is made to be relatively great.
PCT/NL96/00154 and PCT/NL96/00153, which are in the name of the applicant, have disclosed a method for direct multiple impact, in which the impact face is formed by a planar armoured ring which is disposed around the rotor and can be rotated in the same direction and at the same angular velocity as the rotor, around the same axis of rotation; furthermore, its impact face, which is directed inwards, has a conical shape which widens downwards. The material, which after the first impact still has a considerable residual velocity, is guided further to a stationary second impact plate or bed of the same material, where it undergoes the second impact. When seen from a co-rotating position, i.e. when seen from a viewpoint which moves together with the rotor, primarily the radial velocity component is active at the moment that the grain comes off the delivery end of the guide. The transverse velocity component of the material to be broken is in fact at that moment equal to that of the delivery end. After the material to be broken comes off the delivery end, it bends off gradually, when seen from a viewpoint which moves together with the rotor, in a direction towards the rear, when seen from the direction of rotation, thus describing a spiral path. In the known method for direct multiple impact, the impact face is directed perpendicular to the radius of the rotor shaft and therefore has to be disposed at a relatively short radial distance from the delivery end of the guide, because, if this distance becomes too great, the angle at which the material to be broken strikes the horizontal face becomes too oblique, with the result that the impact intensity decreased considerably and the wear increases considerably. The short distance required is the cause of the impact velocity against the co-rotating impact face being defined primarily by the radial velocity component. In order to generate a reasonable radial velocity component, the guide on the rotor blade has to be made relatively long, or else the angular velocity has to be raised considerably, which in both cases leads to a high level of wear to the guide and extra power consumption. Since the transverse component does not contribute to the impact intensity, or does so only to a limited extent, a not insignificant part of the energy supplied to the material to be broken is not used profitably during this first impact. However, the unused energy to a large part remains after the first impact, and in the known method for multiple impact is utilized during one or more immediately following impacts against stationary impact faces.
SU 1,248,655 has disclosed a device in which an impact means is situated outside the rotor, in line with the guide, the centre of the radial impact face of which impact means is directed perpendicular to the radius which joins this centre to the centre of the rotor, which impact face can be rotated at the same velocity as the rotor around the axis of rotation. The impact face is in this case disposed at a relatively short radial distance beyond the delivery end of the guide, since, if the radial impact face were to be disposed at a greater distance beyond the guide, the material to be broken would pass along the back of the impact face, when seen in the direction of rotation. The relatively short distance between the delivery end and the impact face has the consequence that the transverse velocity component scarcely contributes to the impact intensity, as a result of which, since the residual energy in this known method is not utilized further in the first impact, a large proportion, approximately half, of the energy supplied to the material to be broken is completely lost.
FR 2,005,680 has disclosed a direct multiple impact crusher, in which the rotor is equipped with guides which in relative terms are very short and are disposed close to the axis of rotation. In this case, the material is not metered centrally onto the rotor blade, but rather directly above the guides, from where it is flung outwards, whereupon the material is taken up by a large number of short radial impact faces which are mounted along the edge of the rotor blade. A large number of short, radially directed, stationary impact faces are disposed directly around these guides, resulting in a sort of grinding track. The conveyance of the grains between these impact faces is given extra impetus with the aid of an air flow. A problem with the known device is that there is a considerable disturbing effect during the entry of the material at the location of the top edges of the short guides, with the result that the impact acceleration is extremely chaotic, and also that there is a considerable disturbing effect at the location of the points of the co-rotating impact faces.
JP 54-104570 (U.S. Pat. No. 4,373,679) has disclosed a direct multiple impact crusher, in which the material is metered into a thin-walled cylinder which is located on the central part of the rotor blade, from where the material is flung outwards through slot-like openings in the cylinder wall, under the effect of centrifugal force. Impact members are fastened along the edge of the rotor at some distance outside the cylinder. These impact members are preferably formed by pivoting hammers. The cylinder structure with the slot-like opening is selected so as to minimize the length of the impact faces, so that the grains are not accelerated radially, but rather, with an impact, are guided outwards from the cylinder in an essentially tangential path only under the effect of the transverse velocity component (tip velocity). The aim of the method is to guide the material outwards always in an essentially tangential--i.e. essentially the same--direction, irrespective of the rotational speed of the rotor. It is stated that if the grains are guided outwards in a tangential path of this kind, the movement of the grains, even those with a relatively small diameter, is not affected by turbulence caused by the rotating hammers. Furthermore, the tangential path makes it possible to control the location where the grains strike the co-rotating hammers, by turning the cylinder with respect to the hammers. The known crusher has a number of drawbacks. The material which is metered onto the centre of the rotating rotor blade on the bottom of the cylinder describes, when seen from the slot-like opening in the cylinder wall, an outwardly directed spiral (Archimedes' spiral) path in a direction opposite to the direction of rotation of the rotor. In doing so, the material develops, with respect to the slot-like opening, only a low speed. It is therefore inevitable that part of the material will pass through the slot-like opening without coming into contact with the edge of the slot-like opening, i.e. will, as it were, roll outwards through the gaps. Some of the material comes into contact with the edge and in so doing is accelerated by means of an impact, in which case the material can be hit by the points or by the short impact face, or by the very short impact face. A significant problem with the crusher according to the invention is that since the material is unable to develop any radial velocity component, or can develop only a very limited radial velocity component, the flow rate of the said rotor blade, which is essentially a function of the radial velocity component, is limited. This was pointed out earlier in the discussion of cylindrical guide members of this kind. Furthermore, the feed of the material to the slot-like opening is disturbed to a considerable extent, due to the fact that, under the effect of centrifugal force, material becomes attached to the cylinder segments between the slot-like openings, with the result that bridges are formed in the cylindrical space. Only a limited amount of the grains will really hit the impact face of the hammers full on, with the impacts taking place spread along the impact face. Moreover, since there is no protective (tip) structure provided, the edge will become worn very quickly and irregularly, with the result that the way in which the grains are guided outwards is disturbed further. In order nevertheless to subject all the grains to an impact, a second set of hammers is provided which are mounted along the edge of the rotor blade, in a plane directly below the first hammers.
EP 0,562,163 has disclosed a symmetrical multiple impact crusher in which the rotor blade is equipped along the edge with hammers, the material being metered from above these hammers and being guided with an impact between stationary impact plates which are directed radially outwards. After striking these plates, the material falls downwards, where it is taken up by a second set of hammers, which rotate along the inside of a steel armoured ring, the opening between the hammers and the armoured ring forming a gap, so that a maximum grain dimension of the broken product is limited.
U.S. Pat. No. 4,145,009 has disclosed a rotor blade which is provided along the edge with hammers, the material being metered around the rotor blade, above the rotating hammers. An armoured ring is disposed around the outside of the hammers, the distance between the hammers and the armoured ring being adjustable, so that the maximum grain dimension of the broken product can be controlled.
In principle, it is possible with direct multiple impact crushers to synchronize the movement of the impact members in such a manner that the grains are always hit full on by the respective impact faces.
U.S. Pat. No. 1,331,969 has disclosed a multiple synchronized impact crusher in which the moving impact plates are mounted on two rotors which are situated next to one another and rotate about horizontal shafts, the rotating movement of the rotors being mutually adapted so that the material is successively hit firstly full on by the first impact plate and immediately afterwards full on by the second impact plate.
EP 0,583,515 has disclosed a device for direct multiple (double) impact, in which the material is comminuted by a first impact plate which rotates around a first axis of rotation and from which the material is guided in a direction towards a second impact face, which rotates about a second axis of rotation and the rotating movement of which is synchronized with that of the first impact face in such a manner that the material is hit full on twice immediately in succession. A problem with the known method is that the direction in which the material is guided from the first impact face inevitably exhibits a certain dispersal, with the result that this material is hit by the second rotor blade at "considerably" differing distances and thus at "considerably" differing tip velocities of the axis of rotation. It is claimed that impact against a stationary wall provides the lowest possible loading.
Impact loading is also used for the production of extremely fine material with diameters of less than 100 .mu.m and even 100 .mu.m. Since the movement of fine material is affected to a considerable extent by the air resistance, the rotor therefore has to be disposed in a chamber in which there is a vacuum. To break fine material (powder) by impact loading to give an extremely fine product, the material has to be introduced at a very great velocity, which places high demands on the structure whose rotor blade has to rotate at a very high speed, while a high level of wear is found on the means by which the material is accelerated.
U.S. Pat. No. 4,138,067 has disclosed a single impact crusher in which the material is flung outwards with the aid of a rotor, which is provided with closed guide ducts, into a chamber in which there is a vacuum and in which a stationary armoured ring is disposed around the outside of the rotor. Other centrifugal vacuum impactors have been disclosed in U.S. Pat. No. 4,645,131 and U.S. Pat. No. 4,697,743 and U.S. Pat. No. 4,738,403.
EP 0 750 944 discloses an vacuum low temperature impact system.